Generally, a conversion device that outputs AC power based on a DC power supply includes: a step-up circuit which steps up voltage of the DC power supply to a constant level equal to or higher than a required AC peak value and outputs the stepped-up voltage to a DC bus; and an inverter which modulates the constant voltage of the DC bus so as to have an AC waveform through switching, and outputs the modulated voltage. The step-up circuit and the inverter each perform high-frequency switching constantly. Due to the high-frequency switching, the corresponding switching loss occurs, and this causes deterioration in conversion efficiency. In order to reduce such switching loss to enhance the conversion efficiency, the present inventors have proposed a conversion device that minimizes the number of times of high-frequency switching (see Patent Literature 1).
A basic operation of this conversion device is as follows: based on comparison between a required AC waveform and voltage of the DC power supply, when the voltage needs to be stepped up, the step-up circuit performs a step-up operation to form the AC waveform, and when the voltage need not be stepped up, the step-up operation is stopped and the inverter performs high-frequency switching. Thus, the number of times of high-frequency switching can be decreased as a whole.